There are disclosed an aqueous sol of zirconium oxide obtained by a method including: baking colloidal zirconium oxide having a specific surface area of 10 to 400 m2/g and a particle diameter (measured by a dynamic light scattering method) of 20 to 500 nm and containing dewaterable moisture of 4 to 15% by weight, at a temperature of 400 to 1000° C. for 0.05 to 50 hours; adding to the zirconium oxide obtained by the baking, a water-soluble acid or a water-soluble alkali; and wet-grinding the resultant zirconium oxide; wherein the particle diameter (measured by a dynamic light scattering method) of the obtained aqueous sol of zirconium oxide is one to three times the particle diameter of the colloidal zirconium oxide before the baking, and a production method thereof (Patent Document 1).
Also, there is disclosed a polishing agent containing zirconia and a poly acid such as a polyacrylic acid (Patent Documents 2 and 3).
Also, there is known a method including: neutralizing an aqueous solution containing a water-soluble zirconium salt such as zirconium nitrate, zirconium sulfate and zirconium oxychloride with ammonia water to precipitate zirconium hydroxide; filtrating and washing with water the obtained precipitate; and drying and calcining the precipitate, or a method including: heat-hydrolyzing an aqueous solution of a water-soluble zirconium salt to generate a sol; and drying and calcining the sol (Non-Patent Document 1).
Further, there is known a method including: heat-hydrolyzing an aqueous solution of a water-soluble zirconium salt to generate a sol; and drying and calcining the sol (Non-Patent Document 2).    [Patent Document 1] JP-A 8-59242 (Claims and Examples)    [Patent Document 2] JP-B 3130279 (Claims)    [Patent Document 3] P-B 3278532 (Claims)    [Non-Patent Document 1] Ceramics Bifummatsu Gijutsu (Ceramics Fine Particles Technology), edited by the Editorial Board of New-ceras Series, pp. 145-153, published by Gakkensha Co., Ltd., 1994    [Non-Patent Document 2] Inorganic Chemistry (Inorg. Chem.), Third Edition, p. 146, 1964